When measuring heat deflation temperature, we put the sample on two support points, and put specific gravity on the middle, making the internal stress of the most peripheral part 0.46Mpa (66 psi )or 1.82 Mpa (264 psi) around the middle spot, the temperature of the time is called heat deflection temperature.

It is wrorg that some people think that heat deflection temperature is the maximum operating temperature of the finished product made of the material, as the temperature merely briefly displays relation between plastic material temperature and temperature chnge, and the relation changes as temperature varies at different time. When heating time and pressure onto the samples change, the relation changes too. Furthermore, HDT only relates to heat resistance in short time, not long time heating. If we want to know heat resistance of a plastic material, we need to have a heat aging test.

Such being the case, HDT is mainly used to compare heat resistance of plastic materials under short time heating.